In the process for soldering the cooling assembly, the cooling pipes, through which the cooling medium flows, are coated with a film of a fusible alloy prior to being fitted with the cooling fins, which are arranged on the pipes in an array. The assembly is then immersed in the thermal etching bath where the fusible alloy or solder melts, coating the surfaces of the elements of the assembly and fusing them together.
In practice, it has been found that when the cooling fins are made of aluminum prior to their arrangement on the cooling pipes they must be etched to remove any superficial oxide film which may have formed and then further treated to prevent oxidation and to create a solder-receptive surface.
As an example of the state of the art, the process as practiced by U.S. Pat. No. 3,631,588 etches the aluminum cooling fins for 1-5 minutes in a bath of 40% nitric acid at a concentration of 1.4 g/cm.sup.3, 2% hydrogen fluoride at a 38% concentration, and 58% water, at a temperature of 20.degree.-30.degree. C. Following this treatment, the fin is further treated to provide a protective coating and create a solder-receptive surface by immersion for 30 seconds in a bath of 5-15% stearic acid, 2% ammonium bromide, and 83-93% technical grade alcohol.
Prior to being fitted with the fins, the cooling pipes are coated to a thickness of 0.04 mm with an easily fusible alloy layer of 46% Sn, 15% Cd, 15% Zn, and 23% Pb, or with two successive layers of an easily fusible soldering alloy, the first layer consists of 40% Sn and 60% Pb, while the second layer consists of 51% Sn, 16.2% Zn, 16.2% Cd, and 16.6% Pb.
In the case where the fins are made of copper, brass, or steel, the etching step is unnecessary and the entire cooling assembly is provided with a protective film prior to thermal etching by immersion in a bath of 20% stearic acid, 40% paraffin oil, 18% paraffin, 2% ammonium bromide and 20% diesel oil.
After the above treatments for the aluminum fins or the protective film for the entire assembly, the cooler is immersed for 5-10 minutes in the thermal etching bath consisting of 98% mineral oil and an etching mixture containing 60% stearic acid, 20% ammonium bromide, 10% glycerin, and 10% concentrated ammonia, the bath operating at a temperature between 260.degree. and 300.degree. C. which etches the cooler assembly and melts the fusible alloy coating on the cooling pipes thus effecting the soldering of the fins to the pipes. Subsequent to this operation, the soldering assembly is cooled with an air jet while held in a horizontal position and washed afterward with a suitable solvent.
The problem with the described process and other similar processes involving thermal etching baths for soldering, is the phenomenon of electro-corrosion, which takes place between the treated surfaces of the aluminum fins and the fusible alloy coating provided on the cooling pipes which the known processes do not prevent or even attenuate and which can ultimately cause leaks in the cooler assembly.